Review: location for photosynthesis

Review: light energy to chemical energy

Light reactions: make ATP + NADPH for Calvin cycle


Review: Calvin Benson cycle fixes C and makes sugar

Rubisco - the enzyme that dominates carbon fixation


Carbon in often locked in highly oxidized forms, such as carbonate minerals and carbon dioxide gas. To be useful, as a building block of life, oxidized carbon must be “fixed” into organic forms, rich in carbon-carbon bonds



  • Most CO2 converted into biomass is fixed by rubisco
    • bridge between life and lifeless
    • creates organic C from CO2


  • Most abundant enzyme in the world


  • Large molecule : 16 polypeptides : 8 active sites

Rubisco: the clunky and slow carbon fixer



  • In spite of role, rubisco is remarkably inefficient
    • plant rubisco > cyanobacteria rubisco


  • Only 3-10 reactions per second
    • limits photosynthesis


  • 20% error rate
    • gets worse at high temperatures


  • Hmmm…why is Rubisco so abundant?

Rubisco is a cheater!!!




  • Reacts with both O2 and CO2


  • O2 reaction called Photorespiration
    • uses ATP and NADPH to make CO2
    • totally wasteful!!!


  • Non-specific CO2 targeting related to evolution

Cyanobacteria evolved around 2.7 BYA

C3 photosynthesis how did we get here…




  • Evolution doesn’t always create the best solution


  • Photorespiration is completely wasteful
    • 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2


  • Rubisco did evolve high affinity for CO2 (80x > O2)
    • compare to the composition of the atmosphere…

Rubisco substrate choice

Photorespiration impacts food security

Plants and humanity: Why does photorespiration matter?




  • Consumes 32% of ATP and 28% of NADPH in C3 leaves
    • under current conditions


  • Crop productivity improvement not sufficient to meet 2x food production by 2050


  • Photorespiration decreases US soybean and wheat yields by 36% and 20%
    • loss of 148 trillion potential calories
    • would feed 203 million people for a year

Plant evolution tied to atmospheric shifts in CO2:O2

Why has Rubisco not been replaced?


  • Evolution of enzymes difficult
    • trade-offs between activity and stability


  • Removing oxygen reaction breaks Rubisco
    • selection against modifying mutations


  • RubisCO → RubisC???
    • requires a decrease in fitness
    • plants are stuck with Rubisco


  • Natural selection increased affinity for CO2
    • makes it slow

Evolution tinkers with other parts of photosynthesis



  • Improved and recombined existing parts and pieces
    • work around the photorespiration issue


  • In response to environmental change
    • lower [CO2] & higher [O2]
    • high temperatures
    • water limitations


  • Evolution of new photosynthesis pathways
    • C4 pathway and CAM
    • Allowed plants to exploit new habitats